1. Technical Field
The invention relates generally to integrated circuit (IC) chip packaging, and more particularly, to an IC chip package having automated tolerance compensation.
2. Background Art
Referring to FIG. 1, conventional integrated circuit (IC) chip package 10 includes a chip module 20 mating with a printed circuit board (PCB) 16 via a land grid array (LGA) connector 18. Chip module 20 may include IC chip(s) 12 packaged on a single or multiple chip carrier 14. Chip package 10 further includes a metal stiffener 22, which may have a rigid insulator 24 therein to prevent metal stiffener 22 from shorting the underside of PCB 16. Lid 26 may include a piston and a spar (not shown), which provide for a customized thermal paste gap for individual chips and improved mechanical linkage between lid 26 and chip carrier 14, respectively. Metal stiffener 22 and chip module 20 are coupled together by screws/bolts 32 and springs (not shown) to provide the compression load between chip package 10 and the board necessary to electrically interconnect metal stiffener 22 and chip module 20 via LGA connector 18. The springs may be coil springs or Belleville washers. In alternative arrangements, a heat sink (not shown) may be provided over lid 26.
Ideally, chip package 10 has uniform pressure applied to chip carrier 14 and PCB 16. Although an individual LGA connector 18 contact force requires only 20-60 grams to ensure robust performance over product lifetime, contact force is notoriously non-uniform, especially on a large multichip carrier 14. Most LGA connectors 18 utilize an array of connectors distributed within, and positioned by an interposer (not shown) that is sandwiched between module 20 and PCB 16. Pressure is applied to chip package 10 via screws/bolts 32 and springs (not shown) to mechanically deform the contacts so that sufficient pressure is developed and maintained to establish and maintain electrical connections. In order to assure proper electrical performance, there is a minimum design load that needs to act across each contact of LGA connector 18. Design tolerances and mechanical deflections of components leads to contact force non-uniformity, which may have several causes. First, the retention screws/bolts 32 with springs (not shown) are of necessity outside the periphery of rigid LGA connector 18. The collective contact forces thus tend to flex chip carrier 14, PCB 16 and/or metal stiffener 22, and locally tends to push parts away from each other. This situation also tends to relax contact forces in the center of LGA connector 18. In an alternate load configuration, the load from metal stiffener 22 is near the centerline of module 20, so that the components flex in similar directions. Second, PCBs 16 and chip carriers 14 originate having significant non-planarity, which varies in magnitude and pattern from part to part. Third, PCBs 16 have varying thicknesses. The less uniform the loading, the more total force is required to assure sufficient performance at each contact. For example, to insure that even the lowest force sites reach the minimum required contact force, the total clamping, or retention, force applied by screws/bolts 32 and springs (not shown) must be increased, typically to the equivalent of 90-105 grams per contact (i.e., an overload). In addition, the contact force can vary at different locations on LGA connector 18. In some cases, component tolerance can be so severe that even additional load will not guarantee sufficient load at every contact.
Higher loads increase cost because they require more robust structures to apply them, and these higher loads risk damaging or even breaking the components. In some instances, additional layers are provided in chip carrier 14, to address this problem, but this approach impacts cost and electrical performance. For example, as shown in FIGS. 2 and 3, a polymer bulls-eye structure 36 may be added above rigid insulator 24 to help compensate for flexure by reinforcing the pressure at the center of chip package 10. Polymer bulls-eye structure 36 prevents having to increase the retention force of screws/bolts 32 even further. Unfortunately, polymer bulls-eye structure 26 can only be designed based on particular corresponding PCB 16 structure, and can not automatically conform to PCB 16 process variations. If PCB 16 thickness variation changes, structure 36 may actually make the force distribution worse, even leading to fracture of the module or chip carrier.